1. Field of the Invention
The present invention relates to a non-contact electric power transmission apparatus, and particularly relates to a non-contact electric power transmission apparatus for charging a secondary cell contained in an electronic device, such as a portable device, with a power receiving unit in a non-contact manner.
2. Related Art
In FIG. 7, there are shown in a non-contact electric power transmission apparatus a power supplying unit main body 10 containing a power supplying coil L1 for supplying power and a power receiving unit 50 containing a power receiving coil L50 for receiving power.
The power supplying unit main body 10 has a supporting base 20 for detachably mounting the power receiving unit 50. It is undesirable from the viewpoint of safety if the power supplying unit 10 starts electric power transmission without recognizing that the power receiving unit 50 is mounted in a predetermined position in the supporting base 20.
For example, if a metal piece x such as a coin is near and above the power supplying coil L1 as shown in FIG. 7, continuous electric power transmission heats the metal piece according to the principle of electromagnetic induction, which is dangerous.
In Japanese Patent Laid-Open No. 2000-37047, in order to avoid such heat generation by the metal piece x, the shape of the supporting base of the power supplying unit for placing the power receiving unit, which is a portable device, is elaborated so that the power receiving unit is mounted obliquely. Here, the power supplying coil is arranged in an inclined portion of the supporting base, and the power receiving coil is located on the rear side of the power receiving unit so as to face the power supplying coil. The occurrence of heat generation accidents has been avoided by thus mounting the power receiving unit obliquely.
However, there is a problem that the presence of the metal piece x causes the power supplying coil to displace from the power receiving coil, and the efficiency of the charging is lowered as well as the charging time becomes longer. There is also a problem that the additional design constraints of the power supplying unit make the designing difficult.
Japanese Patent Laid-Open No. 6-311658 discloses a method in which a power receiving coil, which is a power receiving unit, receives electric power by means of intermittently-operated or otherwise restrained electric power transmission from the power supplying side, thereby a recognition signal is sent back to the power supplying unit to be recognized by a recognition circuit, and if it matches, power transmission of regular charging power is started to charge a secondary cell or the like in the power receiving unit.
In this case, heat is not generated when a foreign object (such as a metal) is located since intermittently-operated or otherwise restrained electric power transmission is always performed. However, it is not desirable to always generate electric power uselessly even though it is intermittently-operated or otherwise restrained electric power. In addition, unnecessary electromagnetic waves are radiated in air as long as electric power is transmitted, which is undesirable.